The idea of parallel Earths is one of the most enduring that science fiction has given us, but rarely has it been explored with quite so much gusto as in this new novel by two of the giants of British speculative fiction.

Terry Pratchett has, of course, mainstream cachet, thanks to the phenomenal success of his Discworld comic-fantasy novels and his battle with Alzheimer's. Stephen Baxter, while not quite the same household name, is one of Britain's leading science-fiction writers.

Baxter excels at high-concept "hard SF", while Pratchett is more of a character man with a finely tuned ear for dialogue, and it's this blend of talents that makes The Long Earth such a triumph.

They imagine an infinite chain of identical Earths, just a heartbeat between them. Humans can "step" between these worlds, but until 2015 that ability is the closely guarded preserve of a tiny minority.

Then an inventor puts plans on the internet for a Stepper – a simple device made of easily available components and powered by nothing more wonderful than a potato. The internet being what it is, of course, the first people to put the devices together are children, resulting in a sudden and mysterious exodus of young people into the nearest world along the limitless chain of Earths.

Joshua Valienté lives in a children's home in Madison, Wisconsin, at the time of what soon becomes known as "Step Day". Unlike the other first Steppers, Joshua isn't struck down with 15 minutes of trans-dimensional vomiting upon arrival in our neighbouring Earth, and soon becomes something of a hero by leading scores of children back home. He also realises very quickly that he doesn't need one of the home-made devices to jaunt across the divide – Joshua is a natural Stepper.

By the time Joshua is 28, the so-called Step-wise worlds are a fact of life. Suddenly, there are limitless natural resources such as oil, water, land and gold just one or two steps away in the chain of Earths. A mass migration takes place, families striding off along the chain to establish their own communities.

Joshua is enlisted by what appears to be the digitised consciousness of a dead Tibetan to map the furthest reaches of the Long Earth, travelling more than a million worlds away from our Earth. And while humanity has mysteriously not flourished on any of these worlds except our own, there is life out there ... just not what anyone was expecting.

The Long Earth harkens back to the old SF of the Fifties and Sixties, which revelled in the delight of discovering new worlds. But Pratchett and Baxter have essentially democratised space exploration, taking the joy of finding new Edens out of the hands of rocket-owning millionaires and governments and giving it to the masses. This is an accessible, fun and thoughtful SF novel that offers the potential for a multitude of stories as great as the myriad of Earths.